Undefined index: filename in DataExport.php

Problem/Motivation

When creating a new view a warning occurs. In turn this causes the ajax callback to fail.

Proposed resolution

Use the class' getOption:

        $form['filename'] = [
          '#type' => 'textfield',
          '#title' => $this->t('Filename'),
          '#default_value' => $this->getOption('filename'),
          '#description' => $this->t('The filename that will be suggested to the browser for downloading purposes. You may include replacement patterns from the list below.'),
        ];